1. Field of the Invention
This invention relates to inverters, and more particularly to a two-stage commutation circuit for use in commutating the main thyristors of an inverter.
2. Description of the Prior Art
Inverters are known generally and are devices which transform DC (direct current) electrical energy, such as from a fuel cell or the like, into AC (alternating current) electrical energy suitable for use by utility companies or other consumers of electrical energy. Most inverters include at least one pair of main switching elements connected between the DC energy source and the load. By alternatively actuating each switch, a pulse of electrical energy from the DC source flows through the load first in one direction and then in the reverse direction forming a basic AC waveform.
Numerous different types of switching devices can be employed in an inverter to reverse the current through the load. Semiconductor switches, such as a thyristor, are frequently used in present day inverters and this type of device is typically unidirectional so that the current pulse passes in only one direction from the input terminal to the output terminal when turned on by a suitable signal applied to its control terminal. Some semiconductor switches, as is known, will not immediately change from a conducting to a nonconducting state upon the removal of a control signal from the control terminal but require that the magnitude of the instantaneous current passing from the input terminal to the output terminal be reduced to, and kept at, zero for a predetermined length of time whereupon the semiconductor switch turns off.
The process by which the current is reduced to zero through the semiconductor switch so that it can change from its conducting to its nonconducting state is known as commutation and numerous circuit configurations have been proposed for this function. Many commutation circuits operate by presenting a commutation pulse to the load from a storage device, such as a capacitor or resonant circuit, for a period of greater than the turn off time of the semiconductor switch. Since during this period the load current is supplied by the storage device of the commutation circuit, the magnitude of the current through the semiconductor switch drops to zero for a sufficient period to allow transition to the nonconducting state.
It is well known in the art that the amount of energy stored in the commutation capacitors is a function of the value or capacitance of and the voltage across such capacitors; however, the amount of stored energy required to commutate the semiconductor switches is proportional to the magnitude of the load current, i.e. the greater the magnitude of the load current the more stored energy required to commutate the semiconductor switches. Accordingly, the value of the commutation capacitor is often selected by ascertaining the highest value of load current which must be commutated at the minimum input voltage and then sizing the commutation capacitor such that the necessary commutation pulse can be provided.
A disadvantage of the foregoing method of selection of a commutation capacitor size is that with a single stage commutation circuit, the value of the component parts must be sized to handle the worst case condition, i.e. at maximum load where the input voltage is normally near its lowest level. Accordingly, a large commutation capacitor is required but with such a large commutation capacitor the no-load losses of the inverter are particularly high in that a commutation pulse having a capability for commutating even greater than the full load current is discharged from the storage device during each commutation cycle therefore reducing efficiency.
U.S. Pat. No. 3,805,141 issued Apr. 16, 1974 to Pompa, Jr. et al, assigned to the same assignee as the present application, discloses a commutation circuit for a bimodal inverter which ensures that the main thyristors are commutated during periods of rapidly increasing load current. A single commutation capacitor (item 44) stores electrical energy therein a means is provided for the purpose of reducing the load current flowing through the main thyristors during the commutation period to that required for commutating the semiconductor switch. The time interval between the actuation of the auxiliary commutation circuit and the actuation of the main control rectifier is varied as a function of load in order to ensure that the commutation capacitor is adequately charged to provide a sufficient commutation pulse to decrease the main thyristor current to zero. But at lower levels of load current the time interval between the actuation of the auxiliary commutation circuit and the actuation of the main control rectifier is increased so that the commutation capacitor does not store a significant amount of electrical energy greater than that required to commutate the load current.